Quantifying the effect of dobutamine stress on myocardial Pi and pH in healthy volunteers: A31P MRS study at 7T

Purpose Phosphorus spectroscopy (P-31-MRS) is a proven method to probe cardiac energetics. Studies typically report the phosphocreatine (PCr) to adenosine triphosphate (ATP) ratio. We focus on another(31)P signal: inorganic phosphate (Pi), whose chemical shift allows computation of myocardial pH, with Pi/PCr providing additional insight into cardiac energetics. Pi is often obscured by signals from blood 2,3-diphosphoglycerate (2,3-DPG). We introduce a method to quantify Pi in 14 min without hindrance from 2,3-DPG. Methods Using a(31)P stimulated echo acquisition mode (STEAM) sequence at 7 Tesla that inherently suppresses signal from 2,3-DPG, the Pi peak was cleanly resolved. Resting state UTE-chemical shift imaging (PCr/ATP) and STEAM(31)P-MRS (Pi/PCr, pH) were undertaken in 23 healthy controls; pH and Pi/PCr were subsequently recorded during dobutamine infusion. Results We achieved a clean Pi signal both at rest and stress with good 2,3-DPG suppression. Repeatability coefficient (8 subjects) for Pi/PCr was 0.036 and 0.12 for pH. We report myocardial Pi/PCr and pH at rest and during catecholamine stress in healthy controls. Pi/PCr was maintained during stress (0.098 +/- 0.031 [rest] vs. 0.098 +/- 0.031 [stress]P= .95); similarly, pH did not change (7.09 +/- 0.07 [rest] vs. 7.08 +/- 0.11 [stress]P= .81). Feasibility for patient studies was subsequently successfully demonstrated in a patient with cardiomyopathy. Conclusion We introduced a method that can resolve Pi using 7 Tesla STEAM(31)P-MRS. We demonstrate the stability of Pi/PCr and myocardial pH in volunteers at rest and during catecholamine stress. This protocol is feasible in patients and potentially of use for studying pathological myocardial energetics.